1. Field of the Invention
The present invention relates to cementing compositions and a polymeric additive having phosphonate groups pendant to the polymeric backbone which improves fluid loss control and settling properties, as well as methods of using them.
2. Description of the Prior Art
Hydraulic cements are cements that can set under water. This setting property serves a variety of purposes. Hydraulic cements are often used in placement of pipes or casings within a well bore of a subterranean formation for the construction of oil, gas and water wells. In the oil and gas industry, successful cementing of well pipe and casing during oil and gas well completion requires cement slurries having several important properties. The slurry must have a pumpable viscosity, fluid loss control, minimized settling of particles and the ability to set within a practical time. Polymers, such as hydroxyethyl cellulose, carboxymethylhydroxyethyl cellulose, polyvinyl alcohol and polysulfonated polymers are commonly used to provide these important properties.
In a typical completion operation, the cement slurry is pumped down the inside of the pipe or casing and back up the outside of the pipe or casing through the annular space. This seals the subterranean zones in the formation and supports the casing. The amount of water used in forming the cement slurry depends upon the type of hydraulic cement selected and the job conditions at hand. The amount of water used can vary over a wide range, depending upon such factors as the required consistency of the slurry and upon the strength requirement for a particular job.
These completion procedures often place the hydraulic cement within or next to a porous medium, for example, earthern strata in the well bore. When this happens, water tends to filter out of the slurry and into the strata during placement and setting of the cement. Many difficulties relate to an uncontrolled fluid loss of this type, such as an uncontrolled setting rate, premature gellation of the slurry, bridging of the annular space between the formation and the casing, improper placement of the slurry, impaired strength properties and a contamination of the surrounding strata. These conditions are all undesirable in oil, gas and water well cementing operations.
To lessen fluid loss from the aqueous cement slurry, various of the above mentioned polymeric materials are employed as fluid loss additives. These polymers, while improving certain properties of the slurry, also have attendant disadvantages. For example, these polymers are frequently used at higher concentrations for higher temperatures or with salt to provide adequate fluid loss control. At higher polymer concentrations, the slurry viscosity usually becomes unmanageable, frequently causing high friction pressures during pumping, and its cost effectiveness declines.
The addition of salts such as sodium, potassium or calcium chloride to a cement slurry has several advantages. Salt containing slurries bond more firmly to salt laden intervals often found in subterranean formations. Salts help protect salt formations from leaching or washing out.
The addition of salts, however, has several disadvantages. The effectiveness of fluid loss additives decreases at higher salt concentrations. Therefore, a greater amount of additive is used to maintain effectiveness. However, such increased amounts of additive adversely affect slurry properties since these additives also act as retarders and viscosifiers.
In spite of the recent advances, a need exists for an improved polymeric additive for well cementing compositions which provides a slurry having a pumpable viscosity, adequate fluid loss control, minimized settling of particles and the ability to set within a practical time, both with and without salt.
A need also exists for such an additive which is effective at lower concentrations and over a broader temperature range than currently available additives used to achieve the above listed properties.